This application claims the priority of Japanese Patent Application No. 10-82526 filed on Mar. 12, 1998, which is incorporated herein by reference.
The present invention relates to a lithium secondary battery comprising, as a lithium ion occluding agent for the negative electrode, a substantially amorphous oxide powder (hereinafter sometimes referred to as the "amorphous oxide powder"). More particularly, it relates to improvement of the amorphous oxide powder for the purpose of improving a charge-discharge cycle characteristic of the lithium secondary battery.
A lithium secondary battery can be improved in its voltage and capacity by appropriately selecting a positive electrode material because there is no need to consider a water decomposition voltage. Therefore, in accordance with recent development of portable and/or cordless electronic equipment, lithium secondary batteries have been regarded as a promising driving power supply for such electronic equipment.
As a negative electrode material for a lithium secondary battery, metallic lithium widely used in a lithium primary battery was initially regarded favorable. The usage of metallic lithium, however, was found to sometimes cause an internal short-circuit due to growth of lithium electrolytically deposited into the shape of branches on the surface of the negative electrode during charge.
Accordingly, carbon materials, such as graphite and coke, or oxides, which are free from the aforementioned disadvantage and can electrochemically occlude and discharge lithium ions during charge and discharge, are now used in practical batteries. In particular, graphite having the largest specific capacity among the carbon materials is a representative material used in a practical battery.
However, the amount of lithium ions to be occluded by graphite during charge is limited, and it is theoretically impossible to obtain graphite having a specific capacity exceeding 370 mAh/g. When such graphite is used in the negative electrode material, the resultant lithium secondary battery has difficulty in attaining a capacity sufficiently high for meeting a requirement of elongation of charge intervals.
On the other hand, some of oxides exhibit a considerably large specific capacity in the initial stage of charge-discharge cycles. When the charge-discharge cycles are repeated, however, such oxides can be easily degraded because their structures are broken due to strain derived from an internal stress caused in particles through repeated swelling and shrinkage.
Therefore, as means for stabilizing the structure of such an oxide during charge-discharge cycles, a variety of proposals have been made, for example, to provide an oxide with amorphousness or multicomponent including a combination of various elements.
For example, Japanese Laid-Open Patent Publication No. 7-288123/1995 discloses an amorphous oxide powder including two or more kinds of atoms selected from the group IIIb, IVb and Vb atoms, usable in the negative electrode material for a lithium secondary battery. It is reported that the resultant lithium secondary battery can thus attain a large capacity as well as a good charge-discharge cycle characteristic.
However, in this amorphous oxide powder, the specific capacity is still more largely degraded than that of graphite through repeated charge-discharge cycles, and there has been a demand for further improvement in this regard.